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FuROY helped researchers discover two additional crystal forms of ROY by acting as a molecularly similar seed. The headline’s 2020 count of ten forms is now historical: a 2025 study reported the fourteenth known ROY polymorph, called O22.
What the 2020 discovery showed
ROY is short for 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile. Its name reflects the red, orange and yellow colours of some of its crystals. In July 2020, Chemistry World reported that researchers had used FuROY to help crystallize two additional forms of ROY.
FuROY is a close molecular mimic: it differs from ROY by one atom, with oxygen where ROY has sulfur. Researchers introduced a small mixed ROY–FuROY crystal into either a supercooled ROY droplet or a supercooled ROY suspension in water. The seed helped produce two new ROY polymorphs. The report described the tally as ten, while also noting that another well-characterized form had brought it to eleven.
Those figures belong to the 2020 report, not the present count. A 2025 primary study described O22 as the fourteenth known ROY polymorph, following thirteen forms identified before that work. The 2025 study retained ROY’s record among small molecules in its framing.
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What a polymorph is—and why the distinction matters
Polymorphs are different crystal structures of the same chemical compound. Their chemical composition is unchanged, but their structures—and therefore some physical properties—can differ. Diamond and graphite are a familiar example of materials made of the same element with different structures.
That difference matters in pharmaceutical and specialty-chemical manufacturing, where properties such as melting behaviour and bioavailability can depend on the crystal form. ROY is particularly useful for studying the issue because many forms can be accessed from the same liquid under similar conditions.
Why ROY can form so many crystals
Finding a polymorph is not simply a race to see which structure nucleates first. As Lian Yu explained in a 2010 review, a form that nucleates slowly may still become prevalent if it grows quickly or nucleates on another form. ROY’s many observed forms reflect both thermodynamic stability and kinetic access: the relative stability of structures matters, but so does how readily they nucleate and grow.
That combination makes ROY an unusually rich model for examining how molecular conformation, crystal packing, colour and crystallization behaviour relate. It does not mean researchers can readily predict which other compounds will produce many forms. Yu wrote in 2010, “Despite many studies of ROY, it is still impossible to predict the next molecule that is equally or more polymorphic.” Yu’s review reported seven ROY forms with solved structures at the time; that is another historical snapshot, not a current total.
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What ROY’s colours reveal—and what they do not
ROY’s colours are associated with structural differences, but one mechanism does not explain every form. A 2021 study examined orange-needle ROY using high-pressure single-crystal X-ray measurements. Between ambient pressure and 4.18 GPa, the crystals changed reversibly from pale orange toward dark red, even though the molecule itself deformed only minimally. The authors attributed the colour response to pressure-sensitive intermolecular π-stacking interactions.
The result shows why colour alone is not a complete guide to a polymorph’s structure or behaviour: crystal packing and interactions between molecules can also matter. The 2021 study concerns orange-needle ROY under pressure, so its explanation should not be generalized to every ROY form.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read claims about the ROY record
Any count of known ROY polymorphs needs a date and a definition of what is being counted. A tally can change as new forms are found and as their structures are characterized. For context, the 2020 Chemistry World report first gave a count of ten and then noted eleven well-characterized forms; the 2010 review counted seven forms with solved structures; and the 2025 primary study reported fourteen, including O22.
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These are dated milestones, not competing answers to a timeless question. When comparing forms, scientists can look at crystal structure and molecular conformation, colour and morphology, thermodynamic stability versus kinetic accessibility, and how completely each structure has been characterized. The 2025 report is the latest count established by the sources cited here.
Quick Recap
Sources
- Katrina Krämer, Chemistry World, 29 July 2020, on the FuROY seeding method and the contemporaneous count: 2020 report.
- The 2025 primary study on O22 and fourteen known ROY polymorphs: Crystal Growth & Design.
- Lian Yu’s 2010 review on polymorphism and ROY: Accounts of Chemical Research.
- Warren, McGowan, Renton, Morrison and Funnell’s 2021 study of orange-needle ROY under pressure: Chemical Science.
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